As loss of energy from the earth's interior went on this central core of gas would enlarge until the middle shell was hardly self-supporting. Then, probably at some time of astronomic strain when the earth's, orbit was extra elliptical, it would collapse, in collapsing generate heat, and so stop the process. The collapse would be transmitted to the viscous layer which might be increased, motions set up in it, and so a wrinkling of the outer thin crust on which we live. Then there would be four layers to the earth like the butternut of the figure. First, the inner kernel of gas; second, the hard shell or endocarp; third, a viscous layer like the sarcocarp or pulp, and outside of all the wrinkled crust of exocarp. If such is the structure of the earth we may have in the very structure of the earth itself a reason why from time to time there are collapses of the middle layer leading to elevations of portions of the outer rind, and marking off the chapters in geological history, the lines between geological systems. There are reasons in facts of observation for believing that such is the structure of the earth, of which I have as yet said nothing. We see the interior of a glass marble, I saw the bubble in the interior of Bridgman's glass marble, how? By waves, vibrations, which start from the sun or some other source, and going through it reach my eye. Though the earth is not penetrated by sunlight it is penetrated by the waves and vibrations that start from that jar produced by a crack which we call an earthquake. These vibrations can be received by that eye of the geologist called a seismograph. The seismologist tells us there are three kinds of waves sent out in an earthquake. If you notice the explosion of a blast at a little too close distance you will notice that you see it first, then hear it, and then perhaps a little later a few chips of rock may come flying past your ears. These three things correspond somewhat to the three kinds of waves which spread forth from an earthquake. But in the case of the explosion we see the blast first, then hear later. The waves which produce the sensation of sight are, we know, lateral disturbances, the waves which produce the sensation of sound are waves of condensation, whose motion is in the direction of their propagation and they come later. In the case of the jars of earth, the reverse is true. The first set of waves to arrive are the waves which are due to compression--vibrations in the direction in which the waves are produced--and correspond to sound waves. Later come waves which are transverse sidewise disturbances of the solid mass of the earth. As we can easily see, in an earthquake jar traveling from the opposite end of the earth, there should be no insurmountable difficulty in recognizing the jar, which is a direct upthrow from one which would tilt it to the right or left. Now there is a law of Laplace by which the velocity of spread of sound waves through gas may be calculated. That this